A trade union called Gestha, which groups government tax inspectors and accountants, said in a report Wednesday that under-the-table business dealings grew on average by 15 billion euros annually in the four-year period after the economic crisis started in 2008.

The report said almost 70 percent of cash transactions in Spain involve 500-euro notes, the bill most favored in untaxed transactions.

The report blamed the real estate boom that once fueled the economy and the tripling of the unemployment rate that followed the sector's collapse in 2008.

Spain, with 26 percent unemployment, is struggling to recover from a severe two-year recession.